1. Field
The Invention is in the field of air-breathing, jet propulsion engines and methods of operating same.
2. State of the Art
The basic gas-turbine cycle employed in present air-breathing jet propulsion engines is the Brayton cycle, which is the nearest practical approach to the Carnot cycle regarded as ideal, i.e. no cycle can possibly be more efficient for operation of heat engines. The Brayton cycle includes adiabatic compression, wherein the enthalpy is increaased by mechanical work, constant pressure heating to further increase the enthalpy, and adiabatic expansion wherein a limited portion of the enthalphy is converted to velocity.
Adiabatic compression is thermodynamically a reversible process, i.e. isentropic, and the compressor work is essentially all recoverable as equivalent kinetic energy during expansion of the compressed fluid in a suitable nozzle. The same is not true of thermal energy added to the fluid at constant pressure. Such energy addition is largely an irreversible process and the main cause of low thermodynamic efficiencies in current, air breathing, jet propulsion engines. In those cases in which pressure ratios would require De Laval type convergent-divergent nozzles for complete expansion of the fluid, the recoverable portion of the enthalpy of the fluid in the divergent, i.e. supersonic, part of the nozzle is due almost entirely to compressor work that is non-productive so far as conversion of added heat to work is concerned. The productive portion of the fluid cycle derives from the limited advantage that can be gained by raising the temperature, thereby increasing the critical or acoustic velocity which is the maximum that can be attained in nozzles of the convergent type. This has led to the exclusive use of convergent type nozzles in jet engines and a trend toward ever higher temperatures in order to obtain higher velocities and more favorable ratios between productive and non-productive work. The quantity of heat required to secure a given velocity in this way, however, is always more than that required to obtain equivalent kinetic energy, because the higher the velocity that can be obtained, the higher the exhaust temperature and, consequently, the greater the heat loss. It is common in present jet engines for the energy lost to be twice as much as the energy converted to velocity.
Thermal efficiency of conventional jet engines is increased to a limited extent by raising both the engine pressure ratio and the maximum gas temperature. However, in order to maximize this advantage, compressors designed to achieve very high pressure ratios, of the order of 40:1, are necessary. The maximum temperature of the gases entering the turbine is limited by metallurgical and stress considerations, making the use of costly materials and intricate cooling provisions mandatory. However, in spite of these expediences, the ratio of velocity output to its equivalent heat input is still very poor, i.e. less than 50% at best. This dilemma results from two false premises. The first one is that heat has a variable qualitative property, relating to its ability to convert to work, that increases as the absolute temperature increases. The second one is that entropy is a physical property of heat that absorbs all "low quality" heat (the lower the absolute temperature the lower the quality) and consumes it as a part of the hypothetical heat death. As a consequence of the first premise, current design avoids adding heat to the working fluid at any but the highest possible temperatures. The second premise imposes the condition that "no heat engine can convert all of the heat supplied into work", which is undoubtedly true, and "the maximum amount of heat that can possibly be transformed into work is that of the Carnot cycle", which is undoubtedly false, because the variable qualitative property of heat is made explicit in the Carnot cycle formula.
Experiments with a nozzle of the type described herein show that large amounts of heat can be added to a supersonic stream of air, thereby increasing its velocity without substantially increasing its temperature.